The present invention relates to a semiconductor laser diode and a method of forming the same, and more particularly to a gallium nitride based compound semiconductor laser having a current block layer structure selectively grown for a current confinement and a method of forming the same.
Gallium nitride is larger in energy ban gap than those of indium phosphate and gallium arsenide, for which reason gallium nitride based semiconductors of InxAlyGal-x-yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61) may be applied to light emitting diodes such as semiconductor laser diodes for emitting a light of an wavelength in the range of green light wavelength to ultraviolet ray wavelength.
Gallium nitride based semiconductor may have either hexagonal crystal structure or cubic crystal structure. The hexagonal crystal structure is more stable in energy than the cubic crystal structure.
One of conventional gallium nitride based semiconductor laser diodes is disclosed by S. Nakamura et al. in Extended Abstracts of 1996 International Conference On Solid State Devices And Materials, Yokohama, 1996, pp. 67-69.
The conventional gallium nitride based semiconductor laser diode will be described with reference to FIG. 1. A 300 xc3x85-thick undoped GaN buffer layer 102 is formed on a (11-20)-face sapphire substrate 201. A 3 xcexcm-thick n-type GaN contact layer 103 doped with Si is formed on the 300 xc3x85-thick undoped GaN buffer layer 102. A 0.1 xcexcm-thick n-type In0.05Ga0.95N layer 104 doped with Si is formed on the 3 xcexcm-thick n-type GaN contact layer 103. A 0.4 xcexcm-thick n-type Al0.07Ga0.93N cladding layer 105 doped with Si is formed on the 0.1 xcexcm-thick n-type In0.05Ga0.95N layer 104. A 0.1 xcexcm-thick n-type GaN optical guide layer 106 doped with Si is formed on the 0.4 xcexcm-thick n-type Al0.07Ga0.93N cladding layer 105. A multiple quantum well active layer 107 is formed on the 0.1 xcexcm-thick n-type GaB optical guide layer 106. The multiple quantum well active layer 107 comprises 7 periods of 25 xc3x85-thick undoped In0.2Ga0.8N quantum well layers and 50 xc3x85-thick undoped In0.05Ga0.95N barrier layers. A 200 xc3x85-thick p-type Al0.2Ga0.8N layer 108 doped with Mg is formed on the multiple quantum well active layer 107. A 0.1 xcexcm-thick p-type GaN optical guide layer 109 doped with Mg is formed on the 200 xc3x85-thick p-type Al0.2Ga0.8N layer 108. A 0.4 xcexcm-thick p-type Al0.07Ga0.93N cladding layer 110 doped with Mg is formed on the 0.1 xcexcm-thick p-type GaN optical guide layer 109. A 0.2 xcexcm-thick p-type GaN contact layer 111 doped with Mg is formed on the 0.4 xcexcm-thick p-type Al0.07Ga0.93N cladding layer 110. A p-electrode 112 is formed on the 0.2 xcexcm-thick p-type GaN contact layer 111. The p-electrode 112 comprises a nickel layer laminated on the top flat surface of the 0.2 xcexcm-thick p-type GaN contact layer 111 and a gold layer laminated on the nickel layer. An n-electrode 113 is provided on the recessed surface of the 3 xcexcm-thick n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer laminated on the 3 xcexcm-thick n-type GaN contact layer 103 and an aluminum layer laminated on the titanium layer.
All of the semiconductor layers have hexagonal crystal structure with the (0001)-face grown over the (11-20)-face sapphire substrate 201.
The above conventional gallium nitride based semiconductor laser diode has no current confinement structure, for which reason the above conventional gallium nitride based semiconductor laser diode has a relatively large threshold current.
Other conventional gallium nitride based semiconductor laser diode is disclosed by S. Nakamura et al. in Applied Physics Letters, vol. 69 (1996), p 1477. The other conventional gallium nitride based semiconductor laser diode will be described with reference to FIG. 2. A 300 xc3x85-thick undoped GaN buffer layer 102 is formed on a (11-20)-face sapphire substrate 201. A 3 xcexcm-thick n-type GaN contact layer 103 doped with Si is formed on the 300 xc3x85-thick undoped GaN buffer layer 102. A 0.1 xcexcm-thick n-type In0.05Ga0.95N layer 104 doped with Si is formed on the 3 xcexcm-thick n-type GaN contact layer 103. A 0.5 xcexcm-thick n-type Al0.05Ga0.95N cladding layer 605 doped with Si is formed on the 0.1 xcexcm-thick n-thick In0.05Ga0.95N layer 104. A 0.1 xcexcm-thick n-type GaN optical guide layer 106 doped with Si is formed on the 0.5 xcexcm-thick n-type Al0.05Ga0.95N cladding layer 605. A multiple quantum well active layer 707 is formed on the 0.1 xcexcm-thick n-type GaN optical guide layer 106. The multiple quantum well active layer 707 comprises 7 periods of 30 xc3x85-thick undoped In0.2Ga0.8N quantum well layers and 60 xc3x85-thick undoped In0.05Ga0.95N barrier layers. A 200 xc3x85-thick p-type Al0.2Ga0.8N layer 108 doped with Mg is formed on the multiple quantum well active layer 707. A 0.1 xcexcm-thick p-type GaN optical guide layer 109 doped with Mg is formed on the 200 xc3x85-thick p-type Al0.2Ga0.8N layer 108. A 0.5 xcexcm-thick p-type Al0.05Ga0.95N cladding layer 710 doped with Mg is formed on the 0.1 xcexcm-thick p-type GaN optical guide layer 109. A 0.2 xcexcm-thick p-type GaN contact layer 111 doped with Mg is formed on the 0.4 xcexcm-thick p-type Al0.05Ga0.95N cladding layer 710. The 0.2 xcexcm-thick p-type GaN contact layer 111 has a ridge-shape. A p-electrode 112 is formed on the top portion of the 0.2 xcexcm-thick p-type GaN contact layer 111. The p-electrode 112 comprises a nickel layer laminated on the top flat surface of the 0.2 xcexcm-thick p-type GaN contact layer 111 and a gold layer laminated on the nickel layer. A silicon oxide film is formed which extends on the sloped side walls of the ridge portion of the 0.2 xcexcm-thick p-type GaN contact layer 111 and also on the flat base portions of the 0.2 xcexcm-thick p-type GaN contact layer 111 as well as on side walls of the above laminations of the 3 xcexcm-thick n-type GaN contact layer 103, the 0.1 xcexcm-thick n-type In0.05Ga0.95N layer 104, the 0.5 xcexcm-thick n-type Al0.05Ga0.95N cladding layer 605, the 0.1 xcexcm-thick n-type GaN optical guide layer 106, the multiple quantum well active layer 707, the 200 xc3x85-thick p-type Al0.2Ga0.8N layer 108, the 0.1 xcexcm-thick p-type GaN optical guide layer 109, the 0.4 xcexcm-thick p-type Al0.05Ga0.95N cladding layer 710 and the 0.2 xcexcm-thick p-type GaN contact layer 111. An n-electrode 113 is provided on the recessed surface of the 3 xcexcm-thick n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer laminated on the 3 xcexcm-thick n-type GaN contact layer 103 and an aluminum layer laminated on the titanium layer.
All of the semiconductor layers have hexagonal crystal structure with the (0001)-face grown over the (11-20)-face sapphire substrate 201.
The above ridge structure of the 0.2 xcexcm-thick p-type GaN contact layer 111 might contribute any current confinement for reduction in threshold current. Since, however, a contact area between the p-electrode and the 0.2 xcexcm-thick p-type GaN contact layer 111 is small, a contact resistance of the p-electrode to the 0.2 xcexcm-thick p-type GaN contact layer 111 is relatively large.
Whereas the above ridge structure of the 0.2 xcexcm-thick p-type GaN contact layer 111 is defined by a dry etching process, this dry etching process may provide a damage to the semiconductor layers.
The use of this dry etching process results in complicated fabrication processes for the laser diode.
In the above circumstances, it had been required to develop a novel gallium nitride based compound semiconductor laser and a method of forming the same.
Accordingly, it is an object of the present invention to provide a novel gallium nitride based compound semiconductor laser free from the above problems.
It is a further object of the present invention to provide a novel gallium nitride based compound semiconductor laser having a current block layer structure for a current confinement.
It is a still further object of the present invention to provide a novel gallium nitride based compound semiconductor laser having a reduced threshold current.
It is yet a further object of the present invention to provide a novel gallium nitride based compound semiconductor laser having a reduced resistance to current.
It is a further more object of the present invention to provide a novel method of forming a novel gallium nitride-based compound semiconductor laser having a current block layer structure for a current confinement.
It is still more object of the present invention to provide a novel method of forming a novel gallium nitride based compound semiconductor laser having a reduced threshold current.
It is yet more object of the present invention to provide a novel method of forming a novel gallium nitride based compound semiconductor laser having a reduced resistance to current.
It is moreover object of the present invention to provide a novel method of forming a current block layer structure in a novel gallium nitride based compound semiconductor laser without use of dry etching process.
It is still more object of the present invention to provide a novel method of forming a current block layer structure in a novel gallium nitride based compound semiconductor laser at highly accurate size or dimensions.
The above and other objects, feature and advantage of the present invention will be apparent from the following descriptions.
The primary present invention provides a current block layer structure in a semiconductor device. The structure comprises at least a current block layer of a first compound semiconductor having a hexagonal crystal structure. The current block layer are selectively grown on at least a surface of a compound semiconductor region of a second compound semiconductor having the hexagonal crystal structure by use of dielectric stripe masks defining at least a stripe-shaped opening.
The other present invention provides a method of forming a current block layer structure comprising the steps of providing dielectric stripe masks defining at least a stripe-shaped opening on a surface of a compound semiconductor region having a hexagonal crystal structure, and selectively growing at least a current block layer of a compound semiconductor having the hexagonal crystal structure on the surface of the compound semiconductor region by use of the dielectric stripe masks.
Since the current block layer of a compound semiconductor having a hexagonal crystal structure is selectively grown on a compound semiconductor base region also having the hexagonal crystal structure by use of the dielectric stripe masks defining at least a stripe-shaped opening, for example, a metal organic chemical vapor deposition method, then side walls of the current block layer have a good flatness. The selective growth using the dielectric stripe masks results in the highly flat side walls of the current block layer as compared to when a dry etching process is used. The selective growth using the dielectric stripe masks is superior in size-controllability as compared to when a dry etching process is used. The above selective growth using the dielectric stripe masks is more simple than the dry etching process. It is essential for the present invention that the compound semiconductor of the current block layer has the hexagonal crystal structure and the compound semiconductor base region also has the hexagonal crystal structure and also essential that the current block layer is formed by a selective growth using the dielectric stripe masks defining the stripe-shaped opening. Namely, it is important for the present invention that the current block layer of the hexagonal crystal structure compound semiconductor is formed on the hexagonal crystal structure compound semiconductor base region by a selective growth using the dielectric stripe masks defining a stripe-shaped opening such as a metal organic chemical vapor deposition method. The current block layer selectively grown is capable of casing a current confinement or a current concentration which increases a current density whereby a threshold current is reduced if the above current clock structure is applied to a semiconductor laser.